VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics

  #1  
Old 08-14-2018, 12:31 PM
Tom @ N269CP Tom @ N269CP is offline
 
Join Date: Oct 2017
Location: Green Cove Springs, FL
Posts: 51
Default High Fuel Pressure

My RV-8 is powered by an O-360-A1A with Marvel Schebler carburetor. The fuel pressure gauge is hard piped to the pressure tap on the inlet of the carb. I've been flying it regularly for the past couple of months.

Fuel pressure normally varies between about 3-6 psig during flight on the mechanical pump alone.

When I fired it up yesterday morning to go flying the fuel pressure was reading about 9 psi on the mechanical pump alone at about 1000 rpm. It read about 5-6 psi before starting on the electric booster pump alone. As fouling/stiction is a concern, my A&P checked the inlet strainer to the carb which was clean. We drained the carb bowl and flowed fuel through it to flush any debris.

After reassembly, I fired it up again this morning and the fuel pressure was still indicating 8+ psi at 1000 rpm with the mechanical pump alone. The pressure held steady at 8 psi after shutdown and even after cycling the mixture lever. We bled the panel gauge and the pressure immediately dropped to less than 1 psi. So the 8-9 psi pressure appears to be real.

I'd be grateful for any ideas and suggestions as to what the cause of this anomaly may be.

Thank you.

Tom
Reply With Quote
  #2  
Old 08-14-2018, 01:42 PM
airguy's Avatar
airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
Posts: 4,312
Default

Or the bourdon tube in your gauge is getting weak and is indicating higher than real. You can check that by plumbing in a different gauge and comparing the readings.
__________________
Greg Niehues - VAF 2018 dues paid
Garden City, TX
N16GN flying! http://websites.expercraft.com/airguy/
Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.
Reply With Quote
  #3  
Old 08-14-2018, 02:51 PM
Tom @ N269CP Tom @ N269CP is offline
 
Join Date: Oct 2017
Location: Green Cove Springs, FL
Posts: 51
Default

Quote:
Originally Posted by airguy View Post
Or the bourdon tube in your gauge is getting weak and is indicating higher than real. You can check that by plumbing in a different gauge and comparing the readings.
That's a good thought, Greg. I'm hoping not to have to pull the carb for o/h and want to rule out everything else first. I'll order up a new FP gauge just in case.

The plane is about 16 y/o, so I've been gradually replacing various accessory items which have given up the ghost. I don't believe the carb has ever been off. The mechanical fuel pump is dripping a tiny bit of oil at the lower bolt heads, so that's also on my "soon to replace" list. Love the new PMag I recently installed to replace the original right mag.

All the best,

Tom
Reply With Quote
  #4  
Old 08-14-2018, 05:33 PM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 7,907
Default

Tom, your carb cannot cause an increase in fuel pressure.

Nor can flow restriction raise fuel pressure. Cleaning the inlet screen is never a bad idea, but a plugged screen would have nothing to do with an indicated pressure increase.

Pressure is set by a large pump spring pushing against a diaphragm. Line pressure downstream of the pump is essentially the same at no flow or at WOT fuel flow. The difference between a low pressure pump and a high pressure pump is the size of the spring.

A cam drives a pushrod which pushes against a rocker arm. The other end of the rocker arm pulls upward on a pull rod, which raises the fuel diaphragm, compressing the pump spring. When the cam rotates further and relaxes the pushrod/rocker/pull rod, the spring pushes the diaphragm downward, applying pressure to the fuel in the pump chamber. Note that the spring cannot suddenly become stronger.

There are two check valves, one each at the entrance and exit of the pump chamber. One allows fuel into the chamber, but not out. One allows fuel out, but not back in.

Check your gauge.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #5  
Old 08-14-2018, 10:48 PM
Tom @ N269CP Tom @ N269CP is offline
 
Join Date: Oct 2017
Location: Green Cove Springs, FL
Posts: 51
Default

Quote:
Originally Posted by DanH View Post
Tom, your carb cannot cause an increase in fuel pressure.

Nor can flow restriction raise fuel pressure. Cleaning the inlet screen is never a bad idea, but a plugged screen would have nothing to do with an indicated pressure increase.

Pressure is set by a large pump spring pushing against a diaphragm. Line pressure downstream of the pump is essentially the same at no flow or at WOT fuel flow. The difference between a low pressure pump and a high pressure pump is the size of the spring.

A cam drives a pushrod which pushes against a rocker arm. The other end of the rocker arm pulls upward on a pull rod, which raises the fuel diaphragm, compressing the pump spring. When the cam rotates further and relaxes the pushrod/rocker/pull rod, the spring pushes the diaphragm downward, applying pressure to the fuel in the pump chamber. Note that the spring cannot suddenly become stronger.

There are two check valves, one each at the entrance and exit of the pump chamber. One allows fuel into the chamber, but not out. One allows fuel out, but not back in.

Check your gauge.
Thanks, Dan. That sounds pretty definitive.

I was thinking possibly an issue with the carb float valve. The 5-6 psi on the electric booster pump alone was pretty normal. So to have a significantly higher pressure (9 psi) on the mechanical pump alone seemed anomalous. If the gauge were bad, I'd have expected an anomalously high pressure (2-3 psi higher) on the electric boost pump alone as well. Also, the gauge pressure dropped from 8+ psi to less than 1 psi upon breaking the fitting at the gauge. After stopping the engine by chopping the mixture, which should drain the float chamber, I'd have expected cycling the mixture control, to full rich and back a couple of times, to bleed the pressure to the gauge. But I may be missing something on the internal operation of the carb. Fuel flows during idle and runup to 2000 rpm were normal.

A new mechanical FP gauge is winging it's way to me, so will swap the old gauge out in a few days and will report back.

Kind regards,

Tom
Reply With Quote
  #6  
Old 08-15-2018, 12:02 AM
jjconstant's Avatar
jjconstant jjconstant is offline
 
Join Date: Dec 2006
Location: Oakland CA
Posts: 767
Default

Iíve had 3 VDO fuel pressure senders fail. All failures resulted in an abnormally high fuel pressure reading.
__________________
All Best

Jeremy Constant
RV7A "Stella Luna" ECI IO-360 WW200RV Pmags 360hrs
VAF 2018 paid plus some for those who can't
Reply With Quote
  #7  
Old 08-15-2018, 07:31 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 7,907
Default

Quote:
Originally Posted by Tom @ N269CP View Post
Thanks, Dan. That sounds pretty definitive.

I was thinking possibly an issue with the carb float valve. The 5-6 psi on the electric booster pump alone was pretty normal. So to have a significantly higher pressure (9 psi) on the mechanical pump alone seemed anomalous. If the gauge were bad, I'd have expected an anomalously high pressure (2-3 psi higher) on the electric boost pump alone as well.
A reasonable deduction. I have heard other reports of increased pressure with the low pressure pump. The previous principles of operation are factual, but I won't discount the possibility of some phenomenon not realized.

Quote:
Also, the gauge pressure dropped from 8+ psi to less than 1 psi upon breaking the fitting at the gauge. After stopping the engine by chopping the mixture, which should drain the float chamber...
Mixture control regulates flow from the float bowl, not its fill level. Fill level is controlled by the float valve. Mixture cutoff stops flow, bowl fills, float needle closes and traps fuel in line, with pressure applied by the pump spring/diaphragm.

Cycling the mixture with the engine not running doesn't flow any fuel from the bowl because there is no pressure drop in the venturi. Bowl stays full, float needle closed. Pumping the throttle would eventually drain the system via the accelerator pump, but that would take a long time.

BTW, if I understand correctly, you have a mechanical gauge, not electrical? Jeremy is correct about the VDO senders. Mine is reading 145 psi right now
__________________
Dan Horton
RV-8 SS
Barrett IO-390

Last edited by DanH : 08-15-2018 at 07:34 AM.
Reply With Quote
  #8  
Old 08-15-2018, 02:10 PM
Tom @ N269CP Tom @ N269CP is offline
 
Join Date: Oct 2017
Location: Green Cove Springs, FL
Posts: 51
Default

Quote:
Originally Posted by DanH View Post
A reasonable deduction. I have heard other reports of increased pressure with the low pressure pump. The previous principles of operation are factual, but I won't discount the possibility of some phenomenon not realized.



Mixture control regulates flow from the float bowl, not its fill level. Fill level is controlled by the float valve. Mixture cutoff stops flow, bowl fills, float needle closes and traps fuel in line, with pressure applied by the pump spring/diaphragm.

Cycling the mixture with the engine not running doesn't flow any fuel from the bowl because there is no pressure drop in the venturi. Bowl stays full, float needle closed. Pumping the throttle would eventually drain the system via the accelerator pump, but that would take a long time.

BTW, if I understand correctly, you have a mechanical gauge, not electrical? Jeremy is correct about the VDO senders. Mine is reading 145 psi right now
Ah, okay...my mechanic suggested the mixture valve was located upstream of the float chamber. So this tends to point me back towards the gauge...or possible restriction at the float valve. I really need to study up on the carb internals.

Yes, I have a mechanical FP gauge...the replacement should be here tomorrow.

Cheers!

Tom

Last edited by Tom @ N269CP : 08-15-2018 at 02:14 PM.
Reply With Quote
  #9  
Old 08-15-2018, 02:49 PM
YellowJacket RV9 YellowJacket RV9 is offline
 
Join Date: Oct 2012
Location: Clearwater, FL / KZPH
Posts: 1,140
Default

Myself and several others have had the same problem, caused by a failing mechanical fuel pump. Don't ask me how. But if you confirm the gauge's reading, I don't see much besides a bad pump that would cause high pressure before the carb inlet.

Chris
__________________
Chris Johnson
RV-9A - Done(ish) 4/5/16! Flying 4/7/16

Last edited by YellowJacket RV9 : 08-15-2018 at 02:52 PM.
Reply With Quote
  #10  
Old 08-15-2018, 03:22 PM
kaa kaa is offline
 
Join Date: Dec 2013
Location: Oakland
Posts: 40
Default

My mechanical gauge behaves kind of the same way. It also sometimes shows high readings after the engine has been stopped for a while. I wonder if there is vapor somewhere in the system that heats up? I'm not sure this is a good explanation.

Also, what do these gauges actually measure? Differential pressure w.r.t. atmosphere?
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 12:17 PM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.